In hard-drive systems, the data is stored as magnetic flux regions or “magnets” along the surface of a rapidly spinning magnetic media or hard disk with one or more read/write heads “floating” or “flying” very near the media surface. Given the high data rates and low latency requirements of modern hard-drive read channels, data read from the hard disk is sampled using interleaved sampling techniques. The sampled data is converted by a quantizer, forming an interleaved analog-to-digital converter (ADC) and then processed digitally, or the sampled data is processed with sampled analog techniques, or a combination of both.
Controlling the position of the head along the media uses a repeating pattern of magnets. Unlike data that has a somewhat random pattern, repeating patterns (e.g., 2 T magnet length having a pattern . . . 11001100 . . . ) is readily detectable and produces “tones” or signals having a frequency corresponding to the magnet rate and harmonics thereof.
There are several sensitive measurements in modern hard-drives (for example, flying-height sensing and overwrite measurements) that relies on these repeating patterns. For some of these sensitive measurements, however, it is necessary to resolve a very low level signal in the presence of noise and other large signal interferers. Typically a discrete Fourier transform (DFT) is used to separate out the various tones and then measure the amplitudes of those tones.
For example, determining head flying-height (the height of the read head above the spinning magnetic media in a hard drive) involves the measuring the level of one or more harmonics of the fixed pattern relative to the frequency of the fixed pattern. The difference in amplitude between the frequency of the fixed pattern and the third (or higher) harmonic might be 20 dB or more. For more details on head flying height measurement, see, for example, “Servo Signal Processing for Flying Height Control in Hard Disk Drives” by Boettcher et al., Microsyst Technol (2011) 17:937-944, incorporated by reference herein in its entirety.
Another example is determining the effectiveness of overwriting existing data on a spinning magnetic media by the write head by, for example, writing a high-frequency servo pattern, e.g., a 2 T pattern, and then overwriting that pattern with a lower-frequency servo pattern, e.g., a 7 T pattern, and then measuring the level of the weak residual overwritten pattern in the presence of the large lower frequency pattern to determine the effectiveness of overwriting of the magnetic media. For more details on one example how to perform an overwrite measurement but detecting a low-frequency servo pattern overwritten by a high-frequency pattern, see page 8 of an application note from Tektronix titled “Measurement Solutions for Disk Drive”, document 55 W—17699—1, 2006 and 2009, incorporated by reference herein in its entirety. As in the case for head flying height control, the level of the overwritten pattern in the read signal may be 20 or more dB below that of the higher frequency pattern.
Because the desired signal may be “buried under” other signals and noise, a highly linear sampling system is needed that does not introduce tones or other noise not associated with the desired signals.